The invention relates to the field of cutting by laser, in particular with a view to applications to dismantling and/or taking apart.
The technique of dismantling, or taking apart, by laser is distinguished from laser machining by a certain number of characteristics.
Laser machining is effected using a simple physical phenomenon: light of any colour, with a sufficiently high energy level, applied for a sufficiently long time to a target, causes the latter to melt and vaporises the matter or destructures it (in the case of UV lasers). Laser machining makes it possible to perform manufacturing operations which are generally followed by assembly steps themselves making it possible to implement the laser technique (welding, surface treatment, localised alloying, milling).
Thus production laser machining machines comprising laser sources have been produced and sold mainly by machine tool manufacturers. These machines make it possible to process materials in an optimised manner, which makes the machining operations more precise, more rapid, more repetitive, whilst ensuring a very good level of quality for the machined parts.
These techniques use procedures complementary to the emission and control of the laser light proper, and notably the ejection of added gas at the point of impact of the laser beam, in order to eliminate impurities from the area to be treated, to improve the precision of the cutting, to protect the optical devices situated close to the area of impact of the laser on the material, and to create a gaseous microenvironment in the cutting area in order to control the process.
In the case of dismantling or taking apart by laser, the objects to be achieved for a cutting machine are, a priori, very different. It appears in fact that it suffices, for the dismantling operation to be satisfactory, for the size of the cut pieces to correspond to that required by the packing devices allowing the removal of the said cut pieces.
In addition, it cannot be envisaged, in dismantling devices, using appliances making it possible to send an added gas to the area of impact of the laser beam. This is because such appliances should, if they were used, be situated close to the piece to be cut. However, this cannot be envisaged in the case of dismantling, since the laser firing, for reasons of conditions of operation of demolition sites, must be carried out in compliance with a distance between the lens and the piece to be cut, because of the size of the working area (around 1 to 2 m). The laser machines used for dismantling are therefore not very different, in their operating mode, from those used for production of machining.
In addition, a dismantling laser machine is not intended to work, as in the workshop, under stable conditions, but must rather operate under site conditions: it therefore preferably xe2x80x9cbasicxe2x80x9d, compact, lightweight. Controlling the machine is also, preferably, easier than in the case of a workshop machine.
A cutting device is known through the document U.S. Pat. No. 4 870 244. The teachings of this document are based on the use of a biphotonic effect, that is to say on the emission of a first laser radiation which is continuous, or pulsed and relaxed, which melts the material, followed by the emission of a laser pulse of the xe2x80x9cQ-switchxe2x80x9d type focused on the target constituting the area melted by the first laser, which discharges the material by detonation effect. The use of this method assumes that coordination of the laser pulses is possible and is sufficiently precise to prevent the molten material resolidifying between the two laser pulses, the Q-switch not being able to operate industrially both synchronously and at high power.
According to a variant, this document teaches that the two lasers can be replaced by a single laser operating alternatively in a relaxed mode and in Q-switch switch triggered mode. However, it is clearly stated that, because of the interval of time necessary between the end of functioning in continuous mode and the start of the pulse, a cooling of the molten material on the target may take place, which limits the possibilities of application of this device, or of this method, to targets requiring only low cutting powers (thickness of material probably less than 0.1 mm for steel).
This document does not therefore describe a device which can be used, in a practical fashion, in industrial dismantling operations, having regard to the conditions of simplicity and compactness which this application requires and which have been measured above: this is because the system with two lasers is complex (it uses synchronisation), whilst the system with a single laser is very tricky to apply and of limited application. In all cases the Q-switch technology limits the power of the process.
In addition, through the Q-switch technique used in this document U.S. Pat. No. 4 870 244, the pulses obtained are difficult to program with regard to time in repetitive mode.
Consequently at the present time a laser cutting device is not known which does not use ejection of added gas on the laser impact area, and which remains, at the same time, simple and flexible in use.
In addition, the current techniques of dismantling by laser which generally operate in the workshop, or in a pilot laboratory with added gas, do not make it possible to effect, without added gas, cutting of material having a great thickness (at least 10 mm). An additional problem is posed because of the presence, during raw cutting, of wastes such as gases, or deposits of the metallic ball type.
The invention therefore proposes a laser cutting system which remains simple in structure and use, compatible with applications in the field of dismantling, using no added gas, and being able to operate at distances of several meters between the lens and the piece to be cut.
In addition, even if, as stated above, the degrees of precision or evenness in the cutting are not, a priori, as critical as in the field of machining, the invention proposes a device for obtaining quality cutting. This has two consequences: it becomes possible to effect cuts with a greater thickness (cm), and it is possible to eliminate secondary waste such as deposits in the form, for example, of metallic balls, and to reduce the volume of aerosols or cutting gases produced.
More precisely, the object of the invention is a device for laser cutting, having:
means for emitting a pulsed laser beam, with energy pulses Exe2x89xa710 joules,
at least one optical fibre for transmitting the laser pulses, from the means of emitting the laser beam in the direction of a piece to be cut,
means of focusing the laser beam, with a focal distance fxe2x89xa750 cm.
Transmission of the beam by means of an optical fibre makes it possible to operate at a distance. In addition, this method does not use the ejection of added gas, at the area of impact of the laser beam. The use of a focusing lens with a large focal distance (fxe2x89xa750 cm or 1 m) make it possible to operate at a distance between the lens and the piece to be cut.
The focusing means include an assembly of lenses or one aspheric lens with an index gradient or a telescope. They can also include a hybrid mixture of diffractive and refractive elements.
The power density necessary with regard to the task of focusing the laser beam used without added gas, in accordance with the invention, can be less than that necessary for a conventional laser cutting.
For focusing distances greater than one meter, not using the diffractive lens, a telescope is used with a centred mirror which has the drawback of high diffraction at the edges of the lenses, but with satisfactory astigmatism, or a decentred mirror, which eliminates the diffraction but focuses the beam elliptically along two axes.
Using a laser operating in pulse mode makes it possible to create a high-energy pulsed plasma in the vicinity of the impact area on the piece to be cut. It is because the consequence of the laser pulse is to add to the heating of the material a blast effect which makes it possible to omit the assistance gas used in conventional industrial applications, or the second Q-switch described in U.S. Pat. No. 4 870 244. This blast effect is explained by the quality of the plasma, with high energy, created by the laser pulses.
This plasma has another positive effect on the cutting: it allows autofocusing of the beam in the thickness of the material cut. This explains why, in spite of the absence of assistance gas, cuts of very good quality are obtained (the kerves are very fine, with a size substantially identical to the diameter of the focal spot, and with parallel edges) and deep (it becomes possible to cut thick materials). Obtaining fine kerves also makes it possible to reduce the secondary waste produced, such as the gases or deposits of the metallic ball type, and therefore to avoid the rapid dirtying of the focusing means present at the output of the optical fibre.
The invention preferably uses a single laser mode (relaxed mode) which makes it possible to obtain modal stability, a garantee of the uniformity of the coupling for the cutting even on a site.
Control of a laser in pulse mode is easy: the height, width and spacing of the pulses are adjustable. It is therefore possible to optimise the cutting operations according to the context, given the effect of these pulse parameters on the spatio-temporal physics of the plasma created in the vicinity of the impact area. For example, if the cutting is effected on small thicknesses, it is not necessary to send very high energy pulses.
Means of focusing the laser beam are situated at the end of the optical fibre or fibres.
The end of the optical fibre intended to convey the laser energy to the part to be cut can form part of a cutting head which may be orientable.
Means can also be provided for assessing the distance between the cutting head and the piece to be cut. These means can include, for example, a camera, and/or a range finder and/or a profile meter.
It is also possible to provide means of controlling the position of the cutting head and/or means of controlling the focusing of the beam.
Moreover, means can be provided for moving the cutting head. Thus, in the case of a piece to be cut which it is not possible to move, it is the end of the cutting head itself which is moved. These movement means can for example include one or more robotised arms, or movement devices with one or two axes whose speed and position are servo-controlled.
The means emitting a laser beam which can be used in the context of an application of the dismantling type are preferably lasers of the YAG-Nd or iodine-oxygen type.
Another object of the invention is a method for the laser cutting of a piece to be cut including:
the emission of laser pulses, with an energy of Exe2x89xa710 joules,
the transmission of these laser pulses by at least one optical fibre, in the direction of a piece to be cut,
the focusing of the laser beam onto the said piece to be cut, by means of a focusing device with a focal distance fxe2x89xa750 cm.
This method makes it possible to resolve the problems which have already been discussed above, with the same advantages as those mentioned above in relation to the description of the device.
According to a particular embodiment of this method, the distance between the piece to be cut and the end of the cutting head intended to be turned towards the piece to be cut can be assessed. It is also possible to control the position of the end of the cutting head and/or the focusing of the beam.
Another object of the invention is a method as described above using a device as described above.